In recent years, in order to increase the safety of passengers at the time of a collision, air bag systems are being actively installed in automobiles. The initial air bag systems were a type employing explosive chemicals. This type was expensive, and it could result in environmental pollution or safety problems when automobiles were discarded. Therefore, a hybrid system using an inflator made of a steel filled with an inert gas such as argon gas (referred to in this specification as a “air bag inflator”) along with an explosive chemical was developed as a new type of air bag system. This has been extensively used as an air bag system for passenger seats which allow an increased capacity for the system. A seamless tube manufactured by the so-called Mannesmann process is often used as a tube for an air bag inflator of this hybrid system.
When manufacturing a seamless tube by the Mannesmann process, first, a raw material in the form of a billet is heated to 1150-1280° C. in a rotary hearth heating furnace. The billet is then subjected to piercing using a plug and grooved rolls of a piercer to produce a hollow shell body. The hollow shell body with a mandrel bar inserted inside it is elongated by an elongator to form into a mother tube while the outer surface of the hollow shell body is constrained by the grooved rolls of the elongator usually having 5 to 8 stands. The wall thickness of the hollow shell body is thereby reduced to a predetermined value. The mandrel bar is then pulled out of the resulting mother tube, and after the mother tube is reheated to a temperature of 850-1100° C. in a reheating furnace as required, the mother tube is subjected to sizing in a reducer so as to form a seamless tube having a predetermined outer diameter. In this manner, a seamless tube product is manufactured.
In recent years, due to demands for decreases in weight and the like, there has been a tendency to further decrease the wall thickness of tubes for use as air bag inflators. However, if the amount of deviation in the circumferential direction of the wall thickness of a tube for air bag inflator (referred to below as “thickness deviation”) is large, it becomes necessary to provide a larger allowance for wall thickness, and it is no longer possible to meet demands for decreases in wall thickness. Quality control for a conventional tube for an air bag inflator employs the same wall thickness tolerance as for cold finished tubes for boilers, and it is typical for the tolerance of the wall thickness of such a tube to be in the range of 0-20%.
As disclosed in Patent Documents 1-7, for example, there have been many proposals up to the present time with respect to processes of manufacturing tubes for air bag inflators.
Patent Document 1: JP 10-140249 A1
Patent Document 2: JP 10-140283 A1
Patent Document 3: JP 2001-49343 A1
Patent Document 4: JP 2002-294339 A1
Patent Document 5: JP 2003-171738 A1
Patent Document 6: JP 2003-201541 A1
Patent Document 7: JP 2004-27303 A1